Which of the following tumors is characterized as a solid, complex mass arising from muscle?

Rhabdomyosarcoma is a type of cancer that originates in the soft tissues, specifically from the striated or skeletal muscle. It is known for manifesting as a solid, complex mass, showcasing its muscular origin. In children, it is one of the most common soft tissue sarcomas and often presents in areas such as the head, neck, and retroperitoneum.

This tumor is distinguished by its cellular composition, which includes embryonal rhabdomyoblasts, and can exhibit variances in appearance based on its subtype. The presence of these muscle-derived cells is what characterizes rhabdomyosarcoma as a solid, complex mass, differentiating it from other types of tumors that may arise from different tissue types or not form solid masses at all.

Liposarcoma arises from adipose tissue and primarily consists of lipoblasts; fibrosarcoma originates from fibrous connective tissue, and leiomyosarcoma comes from smooth muscle tissue. None of these are characterized specifically by the presence of skeletal muscle elements, which is a defining feature of rhabdomyosarcoma. Thus, the identification of rhabdomyosarcoma as a solid, complex mass arising from muscle is accurate.